I first saw, heard and then met Rep. Joe Sestak, D-Pa., on the evening of April 21 at the University of Pennsylvania's historic Palestra sports arena. With nearly all 10,000 seats filled the night before the Pennsylvania primary, it was an old-fashioned, rousing rally for Democratic presidential candidate New York Sen. Hillary Clinton.

But I remember one man, dignified but passionate, speaking quietly and eloquently about the obligation of every citizen to do public service for the public good.

He praised Clinton for exemplifying that commitment to public service. He also said nice things about her opponent, then-Sen. Barack Obama, D-Ill., and the fact that it was possible to wage a vigorous campaign for a candidate without tearing down the other person.

Above all, he projected an inner peace and authenticity I hadn't seen very often in all my years in politics. I thought, I have to find out more about this guy.

The first thing I learned was that he was a unique trifecta: a Democrat with a progressive record on most issues, a fiscal conservative, and someone who had a sterling military record and post-9/11 anti-terrorism leadership experience.

Joe Sestak graduated the Naval Academy in 1974 and for 30 years built a remarkable record of leadership and experienced a meteoric rise in national recognition.

Before retiring as a two-star rear admiral, he commanded the USS George Washington aircraft carrier battle group in Afghanistan and Iraq, served as director for defense policy on the National Security Council under President Bill Clinton and, following 9/11, was selected to serve as the first director of "Deep Blue" — the Navy's coordinated anti-terrorism program.

Then in 2006, Sestak ran for Congress in a district that was considered utterly locked up by the incumbent Republican, 10-term Rep. Curt Weldon, from Pennsylvania's 7th District. Since the Civil War, only one Democrat had ever won the seat encompassing Delaware County.

In 2006, Joe Sestak, in a come-from-behind campaign, became the second.

He started out in all the polls behind by 20 points or more. He ended up winning by 12 percent and then, two years later, won again in this essentially Republican district by 20 points. In doing so, he became the highest-ranking former military officer ever elected to Congress.

Now, at the age of 58, Sestak is showing the same pattern in his Pennsylvania U.S. Senate campaign. He courageously decided to challenge the incumbent, Republican-turned-Democrat five-term Sen. Arlen Specter, despite the latter being supported by President Barack Obama, Gov. Ed Rendell, and practically every Democratic elected and party official in Pennsylvania.

I greatly admire and respect Sen. Specter. But he seems to have hurt his campaign when he ran negative campaign ads against Joe Sestak about — of all things — his military record.

Specter's vote against Elena Kagan last year for solicitor general will make it difficult to explain a sudden change of heart when he has to vote for her recent nomination for Supreme Court justice. He just said in the last two days that he now had an "open mind." Being against her before he was for her within a year won't ring true with many Democratic voters.

So Sestak is repeating his come-from-behind story from when he first ran for Congress. He started out in January 2010 down 20 points to Specter. In mid-April, just four weeks ago, Sestak was down 12.

This past Monday, a Rasmussen poll found him up 5 percent, although within the margin of error. But the trend is obvious.

Most significantly, in the general election, the same Rasmussen poll showed Sestak in a virtual dead heat with Republican Pat Toomey, while Specter was down 12 points. So based on that data, it seems Sestak would make a stronger general-election candidate than Specter, which should be no surprise, given Sestak's "purple" combination of social liberalism, cultural-issue moderation and impressive military record.

I am sure that when he wins the primary next Tuesday night, May 18, and I am virtually certain he will, his excitement at the victory will pale in comparison to the joy that he and his wife, Susan, felt when their 8-year-old girl Alexandra fought and survived brain cancer.

It kind of puts the stakes of winning or losing a political campaign in perspective. When I learned about Alexandra and her recovery just recently, it made me understand better where that inner serenity and strength of Sestak that I saw on the night of April 21, 2008, comes from, and why those qualities are bound to make him a great U.S. senator.

Mr. Davis, a Washington D.C. attorney, was Special Counsel to President Clinton from 1996-98 and a member of President Bush's Privacy and Civil Liberties Oversight Board from 2006-07. He is the author of "Scandal: How 'Gotcha' Politics Is Destroying America" (Palgrave Macmillan, 2006).

I first saw, heard and then met Rep. Joe Sestak, D-Pa., on the evening of April 21 at the University of Pennsylvania's historic Palestra sports arena. With nearly all 10,000 seats filled the night before the Pennsylvania primary, it was an old-fashioned, rousing rally for...